The present invention is directed to an omnidirectional antenna having a radiating element that is passively fed with electromagnetic signals by an asymmetrical-shaped pair of cones or discs. The invention is particularly well suited for low-profile antenna applications involving the transmission and reception of data in wireless local area networks.
Low profile antennas are desirable for use in in-building wireless local area network (WLAN) applications. However, it has been technically difficult to balance the requirements for high gain and desirable antenna patterns for in-building communication applications when the antenna is limited to a physically small structure.
Antenna designers appreciate that antenna gain can be improved by placing the radiating element above a large, conductive surface, such as a ground plane. A large ground plane also can support the desired shaping of an antenna pattern. Common design requirements for a ground plane of a low-profile antenna are a conductive material comprising a relatively large surface, typically greater than 5 wavelengths. This conductive material can comprise either a solid surface or a grid having holes of a diameter less than 0.1 wavelength. Although an infinitely large ground plate provides a theoretically ideal conductive surface, conventional low-profile antenna designs often face xe2x80x9creal estatexe2x80x9d constraints. Consequently, low-profile antennas are often limited in their performance by a reduced ground plane size and the limited physical size of a radiating element within the practical constraints of an indoor, workplace environment. For example, a dipole antenna having a direct, active signal feed and constrained by a low-profile configuration can lack sufficient gain to support effective wireless communications in the high multipath environment of a typical indoor WLAN application.
In prior antenna designs, designers have achieved additional gain and desirable radiation patterns by the incorporation of stacked cone and/or disk elements as part of the antenna assembly. Conventional antenna designs have employed cone- or disk-shaped elements that operate in tandem to reflect electromagnetic energy in a manner similar to that of a horn antenna. Other prior antenna designs have used stacked biconical elements to form an array of radiating elements, typically fed by a central coaxial feed or a waveguide distribution network. For example, a discone antenna design has been implemented with stacked vertical, hollow conical elements to eliminate signal reflections and to improve antenna bandwidth. However, these prior antenna designs have not exhibited the physical characteristics required of a low-profile antenna application involving minimal available real estate.
In view of the foregoing, a need exists for a low-profile antenna system for WLAN applications that provides increased gain and more desirable radiation patterns than is possible with existing antenna designs.
The present invention provides significant advantages over the prior art by providing a low-profile antenna to transmit radio frequency (RF) energy with high gain and desirable output patterns, typically for data transmission in an in-building, wireless local area network (WLAN). In general, the present invention is directed to an antenna having an emitter element, such as a dipole, which passively receives a signal feed from a vertically stacked pair of asymmetrically-shaped cone elements. The cone elements or discs form a bicone assembly that is centrally fed by a coaxial cable input at a junction formed by an indirect coupling of the apex of each cone. This inventive antenna assembly can be mounted with a standard wall or ceiling-mounted enclosure, with the low-profile antenna typically extending beneath a metallic enclosure cover that serves as a ground plane.
The present invention generally provides a low-profile, omnidirectional antenna system, employing an asymmetrical bicone design with a passive feed for an emitter element, such as a dipole element. A feed signal can be delivered via a conventional coaxial cable, which centrally feeds a pair of stacked, conductive bicone elements mounted below the dipole element. The coaxial cable is used to distribute electromagnetic energy from a source to the bicone elements, with the center conductor connected to the upper cone and the outer conductive sheath or mesh connected to the lower cone. The bicone elements, which are stacked within the vertical plane of the antenna, are indirectly coupled at a common junction formed by an insulator mounted to the apex of each cone. One or more insulators also can be used to separate the combination of upper and lower stacked cones and a vertically-mounted dipole element. The dipole element is supported within the vertical plane of the antenna by the upper cone. This configuration results in a passive coupling of electromagnetic energy within the vertical plane of the antenna assembly and to the dipole element.
The bicone insulator, which is mounted between the upper and lower cones, can provide the sole mechanical support of the upper cone for one aspect of the present invention. For one aspect of the present invention, the bicone insulator can comprise a threaded insulator of non-conductive material having an internal UNF 4-40 thread and an UNC 10-24 external thread. The female contact receptacle of the bicone insulator accepts the bottom tip of the upper cone and the male contact member fits within an opening of the lower cone to form the common junction between the upper and lower cone elements. The bicone insulator controls the dielectric capacitance between the upper and lower cones. Because the center conductor of the coaxial feed cable passes through an opening in the bicone insulator and into the upper cone, this insulator provides the dielectric loading of a low impedance coaxial transmission line. It will be appreciated that this combination of components for the inventive antenna can be assembled without tools and in the absence of any soldering of the central conductor of the feed coaxial cable to the antenna itself. This supports a low cost implementation of a lower profile antenna for wireless communication applications, such as indoor applications.
For one aspect of the present invention, the antenna can be used in connection with a ceiling-mounted enclosure housing a communications device. In this operating environment, the emitter element of the antenna is typically mounted perpendicular to a conductive enclosure cover operating as a conductive ground plane. Because the enclosure and its cover are typically mounted along the ceiling of an interior location, the mounted antenna points downward toward the interior. The ground plane, which can be provided by a solid or grid-like surface of a metallic ceiling tile, is useful for increasing antenna gain and shaping the beam width within the elevation plane. In particular, the combination of a ceiling-mounted ground plane with the inventive passive feed network for an emitter or radiating element results in an antenna exhibiting a decreased beam width within the elevation plane while exhibiting desirable downtilt beam characteristics. The resulting downtilt radiation pattern is particularly desirable in a ceiling-mounted WLAN application.
That the invention provides an antenna having a bicone assembly for passively coupling electromagnetic energy to and from a dipole element will become apparent from the following detailed description of the exemplary embodiments and the appended drawings and claims.